1. Technical Field
The present invention relates generally to aircraft doors; and, more particularly, to aircraft doors of the type oriented for pivotal movement about a generally horizontal hinge line between a first closed position where the door--for example, a bulk cargo door--lies in a generally downwardly and inwardly inclined plane flush with the plane of the aircraft's fuselage, and a second open position where the door is pivoted inwardly to a generally horizontal position in close proximity to the bulk cargo compartment overhead--i.e., the undersurface of the passenger compartment deck. In its principal aspects, the invention relates to an improved counterbalance system for such aircraft doors--doors which may vary widely in terms of size, weight and location dependent upon the type and size of the particular aircraft within which they are installed--which is characterized by its ability to maintain requisite manually exerted operational forces at levels consistent with the normal capabilities of the average cargo attendant or handler who is required to manually move such doors between closed and open positions.
2. Background Art
Prior to the advent of the present invention, the problems inherent in manual operation of aircraft bulk cargo doors and similar relatively large closure members adapted to pivot inwardly about a horizontal axis have resulted in various proposals for spring actuated counterbalance systems for storing the energy required to counterbalance the changing gravitational moment created as such a door is pivoted about its horizontal hinge axis between a first closed position wherein the door lies in a downwardly and inwardly inclined plane where hinge moments are of least consequence, and a second open position where the door lies in a generally horizontal plane, thus producing significant gravitational hinge moments attributable to the weight and size of the door which must be manually overcome by the cargo attendant during a door-opening operation and manually resisted by the attendant during a door-closing operation.
One of the most common and widespread "solutions" to this problem has involved the use of a counterbalance system employing: (i) a horizontal guide tube containing coaxial end-to-end compression springs for storing the energy required to counterbalance the door's gravitational hinge moment; (ii) a double differential pulley; (iii) a first cable coupled at one end to a spring actuator within the guide tube and having its opposite end coupled to one track or groove in the double differential pulley, such groove having a variable pitch radius; (iv) a second cable coupled at one end to the door and having its opposite end coupled to a second track or groove in the double differential pulley, and also having a variable pitch radius; and (iv), a plurality of idler pulleys about which respective ones of the first and second cables are trained. The ratio of the variable pitch radii in the double differential pulley and the magnitude of spring load are tailored to provide the desired counterbalance moment to effectively balance or minimize the gravitational hinge moment that must be overcome and/or resisted by the cargo attendant at each different position of the cargo door as it is shifted between its fully opened and fully closed positions. In terms of "solving" the gravitational hinge moment problem, per se, the foregoing counterbalance system has proven highly effective and is in use today on many large commercial aircraft.
Unfortunately, however, the foregoing counterbalance system is characterized by a number of inherent disadvantages. For example, such systems are relatively complex and, therefore, relatively expensive, requiring two cables, a differential pulley (which is normally oriented in a plane parallel to and closely adjacent the bulk cargo compartment overhead due to serious space constraints), a pair of cables, and multiple idler pulleys. More important, however, the cables are subject to wear at numerous localized regions as a result of cable flexure at those points where the cables are required to wrap about the idler pulleys and the double differential pulley; and, indeed, the system inherently requires that such localized regions of the cables flex several times during each door opening/closing cycle, thus resulting in severe wear and reduced cable life. Of even greater significance is the problem inherent in the double differential pulley arrangement which requires that the cables oscillate as they are "paid on" and "paid off" the variable pitch diameter grooves in the pulley where such oscillation results in misalignment of the cables and pulley grooves, jamming of the counterbalance system, continually changing degrees of wrap about idler pulleys, and resultant wear of the cables and pulleys. Moreover, the space constraints within which such counterbalance systems must operate have heretofore dictated that the idler pulleys and double differential pulley have relatively small diameters--indeed, the idler pulleys are generally only about 2.5 inches in diameter--a factor further contributing to cable wear and shortened cable life.
Yet, notwithstanding the foregoing disadvantages, all of which impose severe maintenance problems and expenses, the two-cable double differential pulley counterbalance system has, until the present invention, presented the most satisfactory approach to the problem and is in widespread use today.
The prior art is, of course, replete with spring actuated counterbalance systems pertaining to a wide variety of closure mechanisms and the like. One such disclosure of interest is that found in U.S. Pat. No. 2,605,494--Lyons, Jr., et al. This system is said to be intended for use as a counterbalance mechanism for hinged hatch covers, bulkhead covers, box covers and the like; and, has been designed for use with outwardly opening covers. The system employs a pair of relatively long brackets mounted on opposite sides of the interior of the cover and which are angularly related thereto; a pair of grooved pulleys mounted on the side panels or bulkheads of the structure; a pair of tension springs, each coupled at one end to the inner surface of the cover; and, a pair of cords or cables secured at one end to the free ends of respective ones of the pair of springs, trained about respective ones of the grooved pulleys, and secured at their opposite ends to respective ones of the brackets. Thus, when the cover is swung to the open position, the relaxed springs tend to hold the cover open; and, during closure of the cover, the operator grasps the cover and pulls it downwardly with sufficient force to overcome the forces imposed by the tension springs.
Other patents of purely incidental interest include, for example, U.S. Pat. No. 4,350,382--Spronck [a balancing system for a foldable platform sideboard rotatable about a horizontal axis]; U.S. Pat. No. 4,165,121--Hori, et al [an actuating mechanism for an outwardly opening side plate on a vehicular loading box]; U.S. Pat. No. 3,561,162--Goldman [a spring actuating mechanism for an outwardly opening hatch door]; U.S. Pat. No. 3,503,092--Welch [a spring actuated counterbalance mechanism for a door rotatable about a vertical hinge axis]; U.S. Pat. No. 3,453,779--Reifenberg [a counterbalance system for an outwardly and downwardly opening appliance door]; U.S. Pat. No. 3,304,670--Logan [a spring counterbalance system for campers and the like having telescopically mounted components]; U.S. Pat. No. 3,303,613--Seuntjens [a spring actuated gate restoring system for gates of the type adapted to be opened by vehicle impact]; U.S. Pat. No. 2,598,888--Capra [a luggage opening system]; U.S. Pat. No. 2,425,365--Davidson [a counterbalance system for holding doors in selected ones of a plurality of positions]; U.S. Pat. No. 2,394,922--Levy [a furnace door balancing system]; U.S. Pat. No. 1,657,466--Frederick [a window operating device employing sash weights and interconnecting linkages]; U.S. Pat. No. 1,594,219--Stockstrom [an oven door operating mechanism]; and, U.S. Pat. No. 1,489,548--Roberts, et al [a spring operated window operating mechanism].
In general, the foregoing patented approaches pertain to counterbalance systems for outwardly opening doors of the type adapted to be pivoted about either a horizontal axis or a vertical axis, or to counterbalance systems for a vertically reciprocable closure member such as a window. While perhaps effective for their intended purposes, they have not been designed for use in connection with the problems inherent with relatively large, heavy, inwardly opening aircraft doors; nor are they capable of solving the problems inherent with such doors. As a consequence, despite the innumerable approaches taken by such prior patentees, prior to the present invention the most commonly accepted and used counterbalance system which has been and is being employed with inwardly opening aircraft doors pivoted for movement about their upper horizontal edges has been the foregoing dual cable, double differential pulley system--this notwithstanding the cable failure problems inherent with such systems.